Expression of cell membrane receptors for urokinase plasminogen activator (u-PA) in the human endometrium increases during the ovarian cycle

Abstract

Human endometrial membranes were found to have binding sites for high molecular weight urokinase plasminogen activator (HMW-u-PA). The binding was characterised and assayed using 125I-HMW-u-PA. The results indicated a receptor, which was highly specific for HMW-u-PA, and that binding was saturable at low concentrations 1.4 (range 1.3–1.6) nmol/1. Scatchard analysis suggested a single class of binding sites with K D 1.1 (range 0.7–1.9) nmol/l. These binding data are similar to those reported in some cell types and cancer tissue. The amount of receptors in individual endometrial samples was assayed either directly, or after exposure of the membranes to pH 2. This treatment removed endogenously bound u-PA and allowed the total number of receptors to be measured. We found the total number of receptors to be higher in the secretory (235 ± 46 fmol/μg protein) than the proliferative (98 ± 16 fmol/μg protein) phase. This was reflected in free as well as occupied receptors. The percentage of occupied receptors was, however, 30–35% in both phases. The increased number of receptors presumably results from the combined action of estradiol and progesterone in the secretory phase.